fix redirect after download when mounted at /slideshow via reverse proxy

Slideshow.py

@@ -6,10 +6,12 @@ import threading
 import time
 import uuid
 from flask import Flask, render_template, request, redirect, url_for, Response
+from werkzeug.middleware.proxy_fix import ProxyFix
 
 from db import init_db, search_images
 
 app = Flask(__name__, static_folder='Pictures', static_url_path='/pictures')
+app.wsgi_app = ProxyFix(app.wsgi_app, x_prefix=1)
 init_db()
 
 # job_id -> {'done': int, 'total': int, 'finished': bool, 'site': str, 'tags': str}